1. Field of the Invention
This invention relates to shell and tube heat exchangers and is particularly concerned with an improved baffle arrangement for the tube bundles of such heat exchangers which results in improved heat transfer, reduces corrosion, and has other advantages.
2. Description of the Prior Art
Shell and tube exchangers are widely used for the indirect transfer of heat from one fluid to another. Typically, such a heat exchanger consists of an external shell having inlet and outlet ports for circulation of the shell side fluid. An elongated bundle of tubes is positioned within the shell and provided with transverse baffles for directing the shell side fluid back and forth across the tubes. The tubes are supported by tube sheets, one of which is normally stationary and the other of which may be stationary or "floating" to accommodate changes in tube length due to thermal expansion. The tube bundle and shell may be arranged so that the tube side fluid makes a single path through the shell or instead makes two or more passes. In a single pass exchanger, the tube side fluid is introduced into a head at one end of the shell and withdrawn from a head at the other end. In a multiple pass unit, the exchanger will generally be provided with an external head containing one or more baffles so that the tube side fluid can be introduced into one portion of the head and withdrawn from the other portion. An internal head within which the tube side fluid flows from one set of tubes into another will generally be located at the other end of the tube bundle. A wide variety of different shell and tube arrangements have been employed in the past.
Most shell and tube heat exchangers used in petroleum refineries and similar process applications are built with the shell side inlet and outlet nozzles located a substantial distance from the inlet and outlet tube sheets because of the necessity for preserving the mechanical strength and integrity of the shell. The length of the shell and flanges, mandatory spacing between welds, nozzle diameters, and other factors establish minimum distances between each tube sheet and the opposite side of the adjacent nozzle. The transverse baffles which direct the flow of shell side fluid back and forth across the tube bundle are usually located so that the first and last baffles are placed adjacent the shell inlet and outlet nozzle openings on the side away from the tube sheets. This results in abnormally wide baffle spacing adjacent the tube sheets where the shell side fluid enters and leaves the exchanger. A "dead" space normally exists adjacent the tube sheet where little circulation of fluid takes place. Experience has shown that corrosion is generally most severe in this area and that relatively little heat transfer takes place at this point. In many exchangers, these problems are accentuated by the use of "floating end support plates". These support plates are installed in exchangers having floating heads and are located in the tube bundles between the floating tube sheet and the shell nozzle in order to support the overhanging end of the tube bundle. Such an arrangement produces another dead space between the tube support plate and the floating tube sheet where essentially no circulation of shell side fluid takes place. Corrosion in this area is often much more severe than at any other place in the exchanger. As a result of the corrosion which occurs here the useful life of tube bundles is often quite short, even though corrosion resistant metals and alloys are normally used. Efforts to avoid these and related problems have in the past been only partially successful.